The invention is broadly useful in the field of hydraulically actuated equipment, being applicable, for example, to excavators which include at least two cylinders containing actuating pistons. It is known in such apparatus to provide separate circuits, each of which includes: hydraulic motor means which may be a cylinder and piston; two pumps; a manual control valve affording control of the position of the piston; and pressure-responsive valve means for unloading one of the pumps in response to high pressure existing in that circuit. In an excavator, as will be understood, the piston of one circuit may control the position of the stick, as during the crowding operation, and the other piston may rotate the bucket in the customary curling and dumping operations. Another pump may provide lateral movement for the assembly comprising bucket, stick and a boom. The tandem pumps mentioned above, or other pumps in a separate circuit may be used to supply two speed track drive, with either manual or automatic upshift and downshift. The invention is described in what follows, as applied to control of stick and swing assemblies.
In such systems it is usual that a single prime mover serves both to move the excavator and to drive several pumps, which supply the pressurized fluid for the pistons of the circuits. The pumps are typically arranged in pairs, and each pair forms part of one of the hydraulic circuits. In this arrangement the power required of the prime mover, for pumping purposes, is the sum of the power required at all pumps. The input horsepower requirement at the prime mover is a function of the flow rate of the pumps, the pressure existing in each circuit, and the overall efficiency of the pumping system.
Generally the pumps are driven at constant speed, and hence the flow rate (gals./min.) is very nearly constant. It follows that the input horsepower required of the prime mover, which is coupled to drive all the pumps, rises linearly with the pressures in the circuits, that is, with the pressures developed in the cylinder-piston actuators. When the pressure in any or all of the various cylinders or other hydraulically operated devices rises substantially, as frequently occurs during a digging operation, the required input horsepower advances toward, and may exceed, the maximum available horsepower. If the maximum is exceeded, the diesel or gasoline motor used as a prime mover will stall.
The prior art includes various arrangements intended to prevent such stalling by unloading one or more pumps, if the pressure in the cylinder circuits approaches a predetermined maximum value, and the present invention provides a novel and particularly advantageous unloading arrangement.